Requirement for caspase activation in monocytic differentiation of myeloid leukemia cells

High caspase 3 and vulnerability to dual BCL2 family inhibition define ETO2::GLIS2 pediatric leukemia

Pediatric acute myeloid leukemia expressing the ETO2::GLIS2 fusion oncogene is associated with dismal prognosis. Previous studies have shown that ETO2::GLIS2 can efficiently induce leukemia development associated with strong transcriptional changes but those amenable to pharmacological targeting remained to be identified. By studying an inducible ETO2::GLIS2 cellular model, we uncovered that de novo ETO2::GLIS2 expression in human cells led to increased CASP3 transcription, CASP3 activation, and cell death. Patient-derived ETO2::GLIS2+ leukemic cells expressed both high CASP3 and high BCL2. While BCL2 inhibition partly inhibited ETO2::GLIS2+ leukemic cell proliferation, BH3 profiling revealed that it also sensitized these cells to MCL1 inhibition indicating a functional redundancy between BCL2 and MCL1. We further show that combined inhibition of BCL2 and MCL1 is mandatory to abrogate disease progression using in vivo patient-derived xenograft models. These data reveal that a transcriptional consequence of ETO2::GLIS2 expression includes a positive regulation of the pro-apoptotic CASP3 and associates with a vulnerability to combined targeting of two BCL2 family members providing a novel therapeutic perspective for this aggressive pediatric AML subgroup.

Non-lethal message from the Holy Land: The first international conference on nonapoptotic roles of apoptotic proteins

Apoptosis is a major form of programmed cell death (PCD) that eliminates unnecessary and potentially dangerous cells in all metazoan organisms, thus ensuring tissue homeostasis and many developmental processes. Accordingly, defects in the activation of the apoptotic pathway often pave the way to disease. After several decades of intensive research, the molecular details controlling the apoptosis program have largely been unraveled, as well as the regulatory mechanisms of caspase activation during apoptosis. Nevertheless, an ever-growing list of studies is suggesting the essential role of caspases and other apoptotic proteins in ensuring nonlethal cellular functions during normal development, tissue repair, and regeneration. Moreover, if deregulated, these novel nonapoptotic functions can also instigate diseases. The difficulty of identifying and manipulating the caspase-dependent nonlethal cellular processes (CDPs), as well as the nonlethal functions of other cell death proteins (NLF-CDPs), meant that CDPs and NLF-CDPs have been only curiosities within the apoptotic field; however, the recent technical advancements and the latest biological findings are assigning an unanticipated biological significance to these nonapoptotic functions. Here, we summarize the various talks presented in the first international conference fully dedicated to discuss CDPs and NFL-CDPs and named 'The Batsheva de Rothschild Seminar on Non-Apoptotic Roles of Apoptotic Proteins'. The conference was organized between September 22, 2019, and 25, 2019, by Eli Arama (Weizmann Institute of Science), Luis Alberto Baena-Lopez (University of Oxford), and Howard O. Fearnhead (NUI Galway) at the Weizmann Institute of Science in Israel, and hosted a large international group of researchers.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

Redox control of protein kinase C: cell- and disease-specific aspects

Hormones, growth factors, electrical stimulation, and cell-cell interactions regulate numerous cellular processes by altering the levels of second messengers, thus influencing biochemical reactions inside the cells. The Protein Kinase C family (PKCs) is a group of serine/threonine kinases that are dependent on calcium (Ca(2+)), diacylglycerol, and phospholipids. Signaling pathways that induce variations on the levels of PKC activators have been implicated in the regulation of diverse cellular functions and, in turn, PKCs are key regulators of a plethora of cellular processes, including proliferation, differentiation, and tumorigenesis. Importantly, PKCs contain regions, both in the N-terminal regulatory domain and in the C-terminal catalytic domain, that are susceptible to redox modifications. In several pathophysiological conditions when the balance between oxidants, antioxidants, and alkylants is compromised, cells undergo redox stress. PKCs are cell-signaling proteins that are particularly sensitive to redox stress because modification of their redox-sensitive regions interferes with their activity and, thus, with their biological effects. In this review, we summarize the involvement of PKCs in health and disease and the importance of redox signaling in the regulation of this family of kinases.

Mutant nucleophosmin deregulates cell death and myeloid differentiation through excessive caspase-6 and -8 inhibition

In up to one-third of patients with acute myeloid leukemia, a C-terminal frame-shift mutation results in abnormal and abundant cytoplasmic accumulation of the usually nucleoli-bound protein nucleophosmin (NPM), and this is thought to function in cancer pathogenesis. Here, we demonstrate a gain-of-function role for cytoplasmic NPM in the inhibition of caspase signaling. The NPM mutant specifically inhibits the activities of the cell-death proteases, caspase-6 and -8, through direct interaction with their cleaved, active forms, but not the immature procaspases. The cytoplasmic NPM mutant not only affords protection from death ligand-induced cell death but also suppresses caspase-6/-8-mediated myeloid differentiation. Our data hence provide a potential explanation for the myeloid-specific involvement of cytoplasmic NPM in the leukemogenesis of a large subset of acute myeloid leukemia.

Can't live without them, can live with them: roles of caspases during vital cellular processes

Since the pioneering discovery that the genetic cell death program in C. elegans is executed by the cysteine-aspartate protease (caspase) CED3, caspase activation has become nearly synonymous with apoptosis. A critical mass of data accumulated in the past few years, have clearly established that apoptotic caspases can also participate in a variety of non-apoptotic processes. The roles of caspases during these processes and the regulatory mechanisms that prevent unrestrained caspase activity remain to be fully investigated, and may vary in different cellular contexts. Significantly, some of these processes, such as terminal differentiation of vertebrate lens fiber cells and red blood cells, as well as spermatid terminal differentiation and dendritic pruning of sensory neurons in Drosophila, all involve proteolytic degradation of major cellular compartments, and are conceptually, molecularly, biochemically, and morphologically reminiscent of apoptosis. Moreover, some of these model systems bear added values for the study of caspase activation/apoptosis. For example, the Drosophila sperm differentiation is the only system known in invertebrate which absolutely requires the mitochondrial pathway (i.e. Cyt c). The existence of testis-specific genes for many of the components in the electron transport chain, including Cyt c, facilitates the use of the Drosophila sperm system to investigate possible roles of these otherwise essential proteins in caspase activation. Caspases are also involved in a wide range of other vital processes of non-degenerative nature, indicating that these proteases play much more diverse roles than previously assumed. In this essay, we review genetic, cytological, and molecular studies conducted in Drosophila, vertebrate, and cultured cells, which underlie the foundations of this newly emerging field.

Effect of 4-hydroxynonenal on Antioxidant Capacity and Apoptosis Induction in Jurkat T Cells

4-Hydroxynonenal (HNE) is one of the major end products of lipid peroxidation and may have either physiological or pathological significance regulating cell proliferation. We studied some biochemical effects of HNE, at various concentrations (0.1-100 microM), on Jurkat T cells incubated thereafter for 24, 48 and 72 h. HNE at low concentrations significantly enhanced the proliferation index, whereas at higher concentrations progressively blocked cell proliferation. Caspase 3 activity increased significantly at HNE concentrations between 1 and 10 microM and decreased at higher concentrations. Superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and glutathione reductase (GSH-Rd) increased progressively with HNE concentrations, particularly GSH-Px. Glucose-6-phosphate dehydrogenase (G6PDH) showed a different pattern, increasing at low HNE (1-5 microM) concentrations and rapidly declined thereafter. These results show that HNE may induce growth inhibition of Jurkat T cells and regulate the activity of typical antioxidant enzymes. Furthermore, the protective effect of doubling the foetal calf serum still points out the risk that cultured cells undergo oxidative stress during incubation.

Ornithine decarboxylase interferes with macrophage-like differentiation and matrix metalloproteinase-9 expression by tumor necrosis factor alpha via NF-kappaB

Ornithine decarboxylase (ODC), a tumor promoter, provokes cell proliferation, and inhibits cell death; but the mechanism involved in cell differentiation remains unknown. Herein, we examine whether it functions during macrophage-like differentiation. Previous studies reveal that ODC, a rate-limiting enzyme of polyamine biosynthesis, and polyamines are involved in restraining immune response in activated macrophage. By using 12-O-tetradecanoylphorbol-13-acetate (TPA)-differentiated human promyelocytic HL-60 and promonocytic U-937 cells, we discover that polyamines block the expression, secretion and activation of MMP-9. Meanwhile conventional expression of ODC represses tumor necrosis factor-alpha (TNF-alpha) expression and nuclear factor-kappaB (NF-kappaB) activation as well as MMP-9 enzyme activity. Following stimulation by TNF-alpha, the secretion of MMP-9 is restored in ODC-overexpressed cells. In addition, the NF-kappaB inhibitors (pyrrolidinedithiocarbamate, BAY-11-7082 and lactacystin) suppress the TPA-induced MMP-9 enzyme activity. Concurrently, both the irreversible inhibitor of ODC, alpha-difluoromethylornithine, and TNF-alpha could not recover MMP-9 activation following NF-kappaB inhibitor treatment in parental cells. Furthermore, ODC could directly inhibit and attenuate NF-kappaB DNA binding and transcriptional activation. Therefore, we suggest that ODC inhibits the TNF-alpha-elevated MMP-9 activation via NF-kappaB as TPA-induced macrophage-like differentiation and this interrupting mechanism may provide a new conceivable resolution why leukemia is poorly differentiated besides atypical growth.

Rapid and selective death of leukemia stem and progenitor cells induced by the compound 4-benzyl, 2-methyl, 1,2,4-thiadiazolidine, 3,5 dione (TDZD-8)

Leukemia is thought to arise from malignant stem cells, which have been described for acute and chronic myeloid leukemia (AML and CML) and for acute lymphoblastic leukemia (ALL). Leukemia stem cells (LSCs) are relatively resistant to current chemotherapy and likely contribute to disease relapse and progression. Consequently, the identification of drugs that can efficiently eradicate LSCs is an important priority. In the present study, we investigated the antileukemia activity of the compound TDZD-8. Analysis of primary AML, blast crisis CML (bcCML), ALL, and chronic lymphoblastic leukemia (CLL) specimens showed rapid induction of cell death upon treatment with TDZD-8. In addition, for myeloid leukemias, cytotoxicity was observed for phenotypically primitive cells, in vitro colony-forming progenitors, and LSCs as defined by xenotransplantation assays. In contrast, no significant toxicity was observed for normal hematopoietic stem and progenitor cells. Notably, cell death was frequently evident within 2 hours or less of TDZD-8 exposure. Cellular and molecular studies indicate that the mechanism by which TDZD-8 induces cell death involves rapid loss of membrane integrity, depletion of free thiols, and inhibition of both the PKC and FLT3 signaling pathways. We conclude that TDZD-8 uses a unique and previously unknown mechanism to rapidly target leukemia cells, including malignant stem and progenitor populations.

Signal transduction pathways that contribute to myeloid differentiation

The production of mature, differentiated myeloid cells is regulated by the action of hematopoietic cytokines on progenitor cells in the bone marrow. Cytokines drive the process of myeloid differentiation by binding to specific cell-surface receptors in a stage- and lineage-specific manner. Following the binding of a cytokine to its cognate receptor, intracellular signal-transduction pathways become activated that facilitate the myeloid differentiation process. These intracellular signaling pathways may promote myelopoiesis by stimulating expansion of a progenitor pool, supporting cellular survival during the differentiation process, or by directly driving the phenotypic changes associated with differentiation. Ultimately, pathways that drive the differentiation process converge on myeloid transcription factors, including PU.1 and the C/EBP family, that are critical for differentiation to proceed. While much is known about the cytokines, cytokine receptors and transcription factors that regulate myeloid differentiation, less is known about the precise roles that specific signaling mediators play in promoting myeloid differentiation. Recently, however, the application of novel pharmacologic inhibitors, siRNA strategies, and transgenic and knockout models has begun to shed light on the involvement and function of signaling pathways in normal myeloid differentiation. This review will discuss the roles that key signaling pathways and mediators play in myeloid differentiation.

Sustained JNK signaling by proteolytically processed HPK1 mediates IL-3 independent survival during monocytic differentiation

We studied monocytic differentiation of primary mouse progenitor cells to understand molecular mechanisms of differentiation. We found a tightly controlled non-apoptotic activation of caspase-3 that correlated with differentiation. Although caspase activity was already detected during monocytic differentiation, a caspase-3 target has not been identified yet. We show that hematopoietic progenitor kinase 1 (HPK1) is processed towards its N- and C-terminal fragments during monocytic differentiation. While HPK1 is an immunoreceptor-proximal kinase in T and B cells, its role in myeloid cells is elusive. Here, we show that the N-terminal cleavage product, HPK1-N, comprising the kinase domain, confers progenitor cell survival independent of the growth factor IL-3. Furthermore, HPK1-N causes differentiation of progenitor cells towards the monocytic lineage. In contrast to full-length kinase, HPK1-N is constitutively active causing sustained JNK activation, Bad phosphorylation and survival. Blocking of caspase activity during differentiation of primary mouse progenitor cells leads to reduced HPK1-N levels, suppressed JNK activity and attenuated monocytic differentiation. Our work explains growth factor-independent survival during monocytic differentiation by caspase-mediated processing of HPK1 towards HPK1-N.

Bergmann glia utilize active caspase-3 for differentiation

Recently, functions associated with caspase have been modified from their well-established role in apoptosis. Although caspases are still regarded as mediators of apoptosis, some of the pro-apoptotic caspases, namely caspase-8, -14 and -3 also regulate differentiation in certain cell types, namely myelomonocytic cells, osteoblasts, skeletal muscle cells, keratinocytes, and T lymphocytes. In the central nervous system, non-apoptotic active caspase-3 expression has been located in proliferating and differentiating neuronal cells of the ventricular zone and external granular layer of the developing cerebellar cortex. We previously demonstrated that active caspase-3 expression was not limited to neuronal cells but also was located in the Bergmann glia of the postnatal cerebellum. In that study, active caspase-3 immunolabeling did not markedly colocalize with Ki67, a proliferation marker, but was present in differentiating Bergmann glia that expressed brain lipid binding protein (BLBP) and thus, by its localization, suggested a role in the differentiation of Bergmann glia. The current study addresses the function of caspase-3 in Bergmann glia development by utilizing a Bergmann glial culture preparation. Inhibition of caspase-3 activity by the peptide inhibitor, DMQD-FMK, increased the number of proliferating precursor glial cells and decreased the number of differentiating Bergmann glia, without significantly altering the non-glial active caspase-3 negative population. The transformation in the developmental state of Bergmann glia occurring after suppression of caspase-3 activity strongly suggests an involvement of this enzyme in promoting differentiation of Bergmann glia.

Increased Hsp27 after androgen ablation facilitates androgen-independent progression in prostate cancer via signal transducers and activators of transcription 3-mediated suppression of apoptosis

One strategy to improve therapies in prostate cancer involves targeting cytoprotective genes activated by androgen withdrawal to delay the emergence of the androgen-independent (AI) phenotype. The objectives of this study were to define changes in Hsp27 levels after androgen ablation and to evaluate the functional relevance of these changes in AI progression. Using a tissue microarray of 232 specimens of hormone-naïve and post-hormone ablation-treated prostate cancer, we found that Hsp27 levels increase after androgen ablation to become highly expressed (>4-fold, P < or = 0.01) in AI tumors. Hsp27 overexpression rendered LNCaP cells highly resistant to androgen withdrawal both in vitro and in vivo. Tumor volume and serum prostate-specific antigen levels increased 4.3- and 10-fold faster after castration when Hsp27 was overexpressed. Treatment of LNCaP tumor cells in vitro with Hsp27 antisense oligonucleotides (ASO) or short-interfering RNA suppressed Hsp27 levels in a dose-dependent and sequence-specific manner increased the apoptotic sub-G0-G1 fraction and caspase-3 cleavage >2-fold, as well as decreased signal transducers and activators of transcription 3 (Stat3) levels and its downstream genes, c-fos and sPLA-2. The cytoprotection afforded by Hsp27 overexpression was attenuated by Stat3 knockdown using specific Stat3 ASO. Coimmunoprecipitation and immunofluorescence confirmed that Hsp27 interacts with Stat3 and that Stat3 levels correlated directly with Hsp27 levels. Hsp27 ASO treatment in athymic mice bearing LNCaP tumors significantly delayed LNCaP tumor growth after castration, decreasing mean tumor volume and serum prostate-specific antigen levels by 57% and 69%, respectively. These findings identify Hsp27 as a modulator of Stat3-regulated apoptosis after androgen ablation and as a potential therapeutic target in advanced prostate cancer.

Functional cytoplasmic domains of the Mac-1 integrin receptor in phorbol ester-treated U937 cells

The integrin receptor Mac-1 regulates adherence and survival of activated tissue macrophages but the underlying molecular mechanisms are poorly understood. Phorbol ester-induced macrophagic differentiation in U937 cells leads to surface expression of Mac-1 and its activation as well. We have attempted to determine essential amino acids for these activities in the cytoplasmic regions of CD11b and CD18 subunits by deletion mutagenesis. There was complete correlation between adherence and survival. Those deletions that lead to loss of adherence and enhanced apoptosis are truncation of CD11b before the MSEGG sequence; CD18 internal deletion of either the membrane-proximal residues before the NPLF sequence or the NPLF sequence itself; CD18 truncation of the C-terminal residues after the NPLF sequence. Unexpectedly, when the NPLF sequence and the C-terminal residues were removed together by truncation, the adherent, antiapoptotic properties were restored. These results were discussed in terms of protein interaction with Mac-1 cytoplasmic regions.

Intracellular signaling mechanisms in photodynamic therapy

In photodynamic therapy (PDT) a sensitizer, light and oxygen are used to induce death of tumor cells and in the treatment of certain noncancerous conditions. Cell death in PDT may occur by apoptosis or by necrosis, depending on the sensitizer, on the PDT dose and on the cell genotype. Some sensitizers that have been used in PDT are accumulated in the mitochondria, and this may explain their efficiency in inducing apoptotic cell death, both in vitro and in vivo. In this review we will focus on the events that characterize apoptotic death in PDT and on the intracellular signaling events that are set in motion in photosensitized cells. Activation of phospholipases, changes in ceramide metabolism, a rise in the cytosolic free Ca2+ concentration, stimulation of nitric oxide synthase (NOS), changes in protein phosphorylation and alterations in the activity of transcription factors and on gene expression have all been observed in PDT-treated cells. Although many of these metabolic reactions contribute to the demise process, some of them may antagonize cell death. Understanding the signaling mechanisms in PDT may provide means to modulate the PDT effects at the molecular level and potentiate its antitumor effectiveness.

Heat shock protein 27 increases after androgen ablation and plays a cytoprotective role in hormone-refractory prostate cancer

Heat shock protein 27 (Hsp27) is a chaperone implicated as an independent predictor of clinical outcome in prostate cancer. Our aim was to characterize changes in Hsp27 after androgen withdrawal and during androgen-independent progression in prostate xenografts and human prostate cancer to assess the functional significance of these changes using antisense inhibition of Hsp27. A tissue microarray was used to measure changes in Hsp27 protein expression in 232 specimens from hormone naive and posthormone-treated cancers. Hsp27 expression was low or absent in untreated human prostate cancers but increased beginning 4 weeks after androgen-ablation to become uniformly highly expressed in androgen-independent tumors. Androgen-independent human prostate cancer PC-3 cells express higher levels of Hsp27 mRNA in vitro and in vivo, compared with androgen-sensitive LNCaP cells. Phosphorothioate Hsp27 antisense oligonucleotides (ASOs) and small interference RNA potently inhibit Hsp27 expression, with increased caspase-3 cleavage and PC3 cell apoptosis and 87% decreased PC3 cell growth. Hsp27 ASO and small interference RNA also enhanced paclitaxel chemosensitivity in vitro, whereas in vivo, systemic administration of Hsp27 ASO in athymic mice decreased PC-3 tumor progression and also significantly enhanced paclitaxel chemosensitivity. These findings suggest that increased levels of Hsp27 after androgen withdrawal provide a cytoprotective role during development of androgen independence and that ASO-induced silencing can enhance apoptosis and delay tumor progression.

Characterisation of Mcl-1 cleavage during apoptosis of haematopoietic cells

Mcl-1 is essential for normal haematopoiesis, being required for lymphocyte development and maintenance. Its role in haematopoietic differentiation and development is associated with its function as an anti-apoptotic member of the Bcl-2 family of proteins although the underlining mechanism is poorly understood. We have characterized caspase cleavage of the Mcl-1 protein during apoptosis. Caspase cleavage resulted in the removal of the PEST regions from the protein and generation of a fragment containing the BH-1, -2 and -3 homology domains. Removal of the PEST regions did not appear to alter Mcl-1 stability, suggesting that these regions are not responsible for Mcl-1's short half-life. In addition, unlike cleavage of Bcl-2 and Bcl-X(L), which resulted in pro-apoptotic fragments, cleaved forms of Mcl-1 were unable to induce apoptosis. This novel regulation of Mcl-1 may have important implications not only for its role in apoptosis but also for the essential role it plays in the differentiation and development of haematopoietic cells.

Active caspase-3 expression during postnatal development of rat cerebellum is not systematically or consistently associated with apoptosis

Development is a dynamic process that includes an intricate balance between an increase in cell mass and an elimination of excess or defective cells. Although caspases have been intimately linked to apoptotic events, there are a few reports suggesting that these cysteine proteases can influence the differentiation and proliferation of cells. Specifically, the active form of caspase-3, which has been classified as an executor of apoptosis, recently has been implicated in a nonapoptotic role in the regulation of the cell cycle, cell proliferation, and cell differentiation. This study investigated the nonapoptotic function and phenotypic expression of active caspase-3-positive cells in the external granule cell layer (EGL) of the postnatal rat cerebellum by using biochemical and immunohistochemical analyses, respectively. Evidence that negates an apoptotic function for the caspase-3-positive EGL cells includes a failure to exhibit chromatin condensation (assessed with TOPRO), phosphatidyl serine externalization (Annexin V labeling), or DNA fragmentation (TUNEL labeling). Proliferative (Ki67-positive) and differentiated (TUJ1-positive) cells within the EGL exhibited a cytosolic expression of caspase-3, whereas terminally differentiated granule cells (NeuN-positive) in the internal granular layer and the migrating granule cells did not express active caspase-3. Thus, this study supports a nonapoptotic role for active caspase-3 in cells residing in the EGL and suggests a possible involvement in EGL proliferation and differentiation.

A Chromatin Immunoprecipitation Screen Reveals Protein Kinase Cβ as a Direct RUNX1 Target Gene

RUNX1 (also known as AML1) is a DNA-binding transcription factor that functions as a tumor suppressor and developmental determinant in hematopoietic cells. Target promoters have been identified primarily through the use of differential expression strategies and candidate gene approaches but not biochemical screens. Using a chromatin immunoprecipitation screen, we identified protein kinase Cbeta as a direct RUNX1 target gene and demonstrate that endogenous RUNX1 binds the chromatinized protein kinase Cbeta promoter of U937 cells. A phylogenetically conserved RUNX1-binding site within the PKCbeta promoter binds RUNX1 in electrophoretic mobility shift analyses and confers RUNX1 responsiveness on a heterologous promoter. Changes in RUNX1 activity affect endogenous protein kinase Cbeta expression, and a dominant-negative form of RUNX1 protects U937 cells from apoptotic stimuli previously shown to be dependent on protein kinase Cbeta. This protection can be reversed by the ectopic expression of protein kinase Cbeta. Together these findings demonstrate that protein kinase Cbeta is a direct, downstream target of RUNX1 and links RUNX1 to a myeloid apoptotic pathway.

The inhibitors of apoptosis: there is more to life than Bcl2

The inhibitor of apoptosis (IAP) genes constitute a highly conserved family found in organisms as diverse as insects and mammals. These genes encode proteins that directly bind and inhibit caspases, and thus play a critical role in deciding cell fate. The IAPs are in turn regulated by endogenous proteins (second mitochondrial activator of caspases and Omi) that are released from the mitochondria during apoptosis. Overexpression of the IAPs, particularly the X-chromosome-linked IAP, has been shown to be protective in a variety of experimental animal models of human neurodegenerative diseases. Furthermore, overexpression of one or more of the IAPs in cancer cell lines and primary tumor samples appears to be a frequent event. IAP gene amplification and translocation events provide genetic evidence that further strengthens the case for classifying the IAPs as oncogenes. Therapeutic strategies that interfere with IAP expression or function are under investigation as an adjuvant to conventional chemotherapy- and radiation-based cancer therapy. This paper reviews the structure and function of the IAP family members and their inhibitors, and surveys the available evidence for IAP dysregulation in cancer.

A decade of caspases

Caspases are a family of cysteine proteases that play important roles in regulating apoptosis. A decade of research has generated a wealth of information on the signal transduction pathways mediated by caspases, the distinct functions of individual caspases and the mechanisms by which caspases mediate apoptosis and a variety of physiological and pathological processes.

Down-regulation of the mitochondrial translation system during terminal differentiation of HL-60 cells by 12-O-tetradecanoyl-1-phorbol-13-acetate: comparison with the cytoplasmic translation system

Mitochondrial (mt) biogenesis depends on both the nuclear and mt genomes, and a coordination of these two genetic systems is necessary for proper cell functioning. Little is known about the regulatory mechanisms of mt translation or about the expression of mt translation factors. Here, we studied the expression of mt translation factors during 12-O-tetradecanoyl-1-phorbol-13-acetate (TPA)-induced terminal differentiation of HL-60 cells. For all mt translation factors investigated, mRNA expression was markedly down-regulated in a coordinate and specific manner, whereas mRNA levels for the cytoplasmic translation factors showed only a slight reduction. An actinomycin D chase study and nuclear run-on assay revealed that the TPA-induced decrease in mt elongation factor Tu (EF-Tumt) mRNA mainly results from decreased mRNA stability. Polysome analysis showed that there was no significant translational control of mt translation factor (EF-Tumt, ribosomal proteins L7/L12mt and S12mt) mRNA expression during differentiation. Thus, the decreased protein level of one of these mt translation factors (EF-Tumt) simply reflects its decreased mRNA level. It was also demonstrated by pulse labeling of mt translation products that the down-regulation of mt translational activity is actually associated with down-regulated mt translation factor expression during cellular differentiation. Our results illustrate that the regulatory mechanisms of mt translational activity upon terminal differentiation (in response to the growth arrest) is different to that of the cytoplasmic system, where the control of mRNA translational efficiency of major translation factors is the central mechanism for their down-regulation.

Phosphatidylserine expression and phagocytosis of apoptotic thymocytes during differentiation of monocytic cells

Expression of phosphatidylserine (PS) on the surface of both macrophages and their apoptotic targets is required for efficient phagocytosis. Monocytes, the precursors of macrophages, do not express PS on their surface and do not efficiently phagocytose apoptotic cells. We report here that PS appears on the surface of both human monocytic U937 cells and primary human monocytes as they differentiate in culture and acquire the ability to phagocytose apoptotic thymocytes. Phagocytosis was blocked by pretreating either the apoptotic target or the phagocyte with annexin V to mask PS and was CD14-dependent. Expression of PS, like other events characteristic of differentiating monocytes such as Mac-1 expression, was independent of the agent used to induce differentiation and was insensitive to the addition of caspase inhibitors. These results demonstrate that PS is expressed on monocytes as part of their differentiation program and is independent of apoptosis.

Prevention of NMDA-induced death of cortical neurons by inhibition of protein kinase Czeta

Excitotoxicity through stimulation of N-methyl-d-aspartate (NMDA) receptors contributes to neuronal death in brain injuries, including stroke. Several lines of evidence suggest a role for protein kinase C (PKC) isoforms in NMDA excitotoxicity. We have used specific peptide inhibitors of classical PKCs (alpha, beta, and gamma), novel PKCs delta and epsilon, and an atypical PKCzeta in order to delineate which subspecies are involved in NMDA-induced cell death. Neuronal cell cultures were prepared from 15-day-old mouse embryos and plated onto the astrocytic monolayer. After 2 weeks in vitro the neurons were exposed to 100 micro m NMDA for 5 min, and 24 h later the cell viability was examined by measuring the lactate dehydrogenase release and bis-benzimide staining. While inhibitors directed to classical (alpha, beta, and gamma) or novel PKCs (delta or epsilon) had no effect, the PKCzeta inhibitor completely prevented the NMDA-induced necrotic neuronal death. Confocal microscopy confirmed that NMDA induced PKCzeta translocation, which was blocked by the PKCzeta inhibitor. The NMDA-induced changes in intracellular free Ca2+ were not affected by the peptides. In situ hybridization experiments demonstrated that PKCzeta mRNA is induced in the cortex after focal brain ischemia. Altogether, the results indicate that PKCzeta activation is a downstream signal in NMDA-induced death of cortical neurons.

Participation of protein kinase C beta in osteoclast differentiation and function

Protein kinase C (PKC) proteins have been shown to be involved in diverse cellular responses of various cell types. In experiments to identify genes regulated during osteoclast differentiation by a cDNA microarray approach, we found that the gene expression of PKC-betaII was upregulated in differentiated cells. Reverse transcription-polymerase chain reaction and Western blotting analyses also showed an increase in PKC-betaI as well as PKC-betaII during osteoclast formation in mouse bone marrow cell cultures in the presence of macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-kappaB ligand (RANKL). Use of an antisense oligonucleotide to PKC-betaII resulted in a reduction in the RANKL-driven osteoclastogenesis. Pharmacological intervention with PKC-beta activity by the specific inhibitor CG53353 suppressed cellular differentiation and fusion processes during osteoclastogenesis and inhibited bone-resorbing function of mature osteoclasts. PKC-beta inhibition abolished the ERK and MEK activation by macrophage-colony stimulating factor and RANKL in osteoclast precursor cells whereas the cytokine-induced NF-kappaB activation was not hampered by the PKC-beta inhibition. Our findings indicate that PKC-beta has a role in regulation of osteoclast formation and function potentially by participating in the ERK signaling pathway of M-CSF and RANKL.

Specific involvement of caspases in the differentiation of monocytes into macrophages

Caspases are cysteine proteases involved in apoptosis and cytokine maturation. In erythroblasts, keratinocytes, and lens epithelial cells undergoing differentiation, enucleation has been regarded as a caspase-mediated incomplete apoptotic process. Here, we show that several caspases are activated in human peripheral blood monocytes whose differentiation into macrophages is induced by macrophage colony-stimulating factor (M-CSF). This activation is not associated with cell death and cannot be detected in monocytes undergoing dendritic cell differentiation in the presence of interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The mechanisms and consequences of caspase activation were further studied in U937 human monocytic cells undergoing phorbol ester-induced differentiation into macrophages. Differentiation-associated caspase activation involves the release of cytochrome c from the mitochondria and leads to the cleavage of the protein acinus while the poly(ADP-ribose)polymerase remains uncleaved. Inhibition of caspases by either exposure to the broad-spectrum inhibitor benzyloxycarbonyl-Val-Ala-(DL)-Asp-fluoromethylketone (z-VAD-fmk) or expression of the p35 baculovirus inhibitory protein or overexpression of Bcl-2 inhibits the differentiation process. In addition, z-VAD-fmk amplifies the differentiation-associated production of radical oxygen species in both phorbol ester-differentiated U937 cells and M-CSF-treated monocytes, shifting the differentiation process to nonapoptotic cell death. Altogether, these results indicate that caspase activation specifically contributes to the differentiation of monocytes into macrophages, in the absence of cell death.

Trophoblast differentiation modulates the activity of caspases in primary cultures of term human trophoblasts

Cultured human cytotrophoblasts are more susceptible than syncytiotrophoblasts to hypoxia-induced apoptosis. Caspases are cysteine proteases that cleave cellular components to effect the apoptotic cascade. We hypothesized that cultured cytotrophoblasts exhibit a higher activity of caspases when compared with syncytiotrophoblasts. Using western analysis, we demonstrated that the pro-caspases 3, 6, 8, and 9 are expressed in cytotrophoblasts cultured for 24 h, and also, in trophoblasts cultured 72 h when syncytiotrophoblasts have formed. Importantly, we found significantly higher activity of all four caspases in trophoblasts cultured 24 h compared with cells cultured 72 h. Colchicine and DMSO, which hinder trophoblast differentiation, enhanced the activity of all four caspases in cells cultured 72 h. Conversely, caspase activity was reduced in trophoblasts cultured for 24 h in the presence of epidermal growth factor, which enhances differentiation. This effect was most pronounced on caspase 3 and was attenuated by addition of the tyrosine kinase inhibitor AG1478. We conclude that cytotrophoblasts exhibit a higher activity of caspases 3, 6, 8, and 9 when compared with the more differentiated syncytium. This may account for the higher susceptibility of cytotrophoblasts to hypoxia-induced apoptosis.

Protein kinase C-beta, fibronectin, alpha(5)beta(1)-integrin, and tumor necrosis factor-alpha are required for phorbol diester-induced apoptosis in human myeloid leukemia cells

The human myeloid HL-60 cell line and its cell variant HL-525 were used to study signaling events leading to apoptosis induction by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC) enzymes. Unlike parental cells, HL-525 cells are PKC-beta deficient and resistant to PMA-induced apoptosis. These cells regain susceptibility to apoptosis induction after transfection with a PKC-beta expression vector. By using this vector and specific neutralizing monoclonal antibodies (mAbs), it was established that PMA-induced apoptosis also called for an interaction between cell-surface alpha(5)beta(1)-integrin and its deposited ligand fibronectin (FN), which is downstream of PKC-beta activation. Experiments with mAbs, the PKC-beta vector, and exogenous FN revealed that the next step entailed an interaction between secreted tumor necrosis factor-alpha and its type I receptor. By using a sphingomyelinase inhibitor, it was concluded that the subsequent step involved ceramide production. Moreover, a permeable ceramide was effective in inducing apoptosis in both HL-60 and HL-525 cells, and this induction was caspase-1 and/or -4 dependent because an inhibitor of these caspases abrogated the induced apoptosis. Based on these and related differentiation studies, we conclude that the above signaling events, the early ones in particular, are shared with PMA-induced macrophage differentiation in the HL-60 cells. It is likely that once these cells acquire their macrophage phenotype and perform their tasks, they become superfluous and are eliminated from the body by a self-triggered apoptotic process that involves our proposed signaling scheme.

The novel triterpenoid CDDO induces apoptosis and differentiation of human osteosarcoma cells by a caspase-8 dependent mechanism

The oleanane triterpenoid 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) is a multifunctional molecule that induces monocytic differentiation of human myeloid leukemia cells and inhibits proliferation of diverse human tumor cell lines. The present studies on human osteosarcoma cells demonstrate that CDDO induces mitochondrial cytochrome c release, caspase-3 activation, and internucleosomal DNA fragmentation. Overexpression of the caspase-8 inhibitor CrmA blocked CDDO-induced cytochrome c release and apoptosis. By contrast, overexpression of the antiapoptotic Bcl-x(L) protein blocked CDDO-induced cytochrome c release, but only partly inhibited caspase-3 activation and apoptosis. In concert with these findings, we demonstrate that CDDO: 1) activates caspase-8 and thereby caspase-3 by a cytochrome c-independent mechanism and 2) induces cytochrome c release by caspase-8-dependent cleavage of Bid. The results also demonstrate that treatment of osteosarcoma cells with CDDO induces differentiation, as assessed by alkaline phosphatase activity and osteocalcin production, and that this response is abrogated in cells that overexpress CrmA. These findings demonstrate that CDDO induces both osteoblastic differentiation and apoptosis by caspase-8-dependent mechanisms.